The invention relates to polyolefin couplers for Joining polyolefin pipes; and to a method of joining such pipes.
Known couplers are widely used for joining pipes of up to 355 mm. Such couplers are of the socket type and have integral heating elements. The heating element melts the inside surface of the coupler and the opposed surfaces of the pipes. The opposed surfaces are generally spaced apart no more than 1-2 mm in the case of smaller couplers but the 355 mm size demands hydraulic fitting of the coupler and pipes. The gap in that case is virtually zero. The molten surfaces merge together and a welded joint is formed between each pipe and the coupler.
The object of the invention is to provide a coupler which can be used to join pipes especially, though not exclusively, pipes of large diameter, for example pipes of 500 mm or larger.
It has been proposed in U.S. Pat. No. 4,775,501 (Raychem Corporation) and in European Patent No. 0197759 (Raychem Corporation) to join polyethylene pipes by a coupler composed of cross-linked polymer containing a particulate conductive filler. The coupler was expanded below its melting point (120.degree. C.). The coupler was recovered to join the pipes by passing a current through the coupler so as to heat it and shrink the coupler onto the pipes. The coupler and the pipes were joined by a fusion joint.
In the two examples quoted in U.S. Pat. No. 4,775,501, the polyethylene pipes had an outside diameter of 0.84 inch (21 mm). The coupler reached its shrinkage temperature in about 30-45 seconds and current was passed into the coupler for a further 1 minute.
The pipe end portions are shown as having shrunk and the end surfaces of the pipes appear to have melted. The result is a considerable reduction in the diameter of the through bore of the pipes.
In the sole example in EP 0197759 the pipes had an outside diameter of 21.3 mm and a wall thickness of 2.8 mm. The heating conditions were as in U.S. Pat. No. 4,775,501. No illustration is given in EP 0197759.
The couplers mentioned in the Raychem patents have to be electrically conductive and to achieve this they are made from a material containing carbon particles in such a concentration that they create on electrically conductive path. It is known that such high loadings of carbon particles can cause a deterioration in desirable mechanical properties of the base polymer.